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Just a post with my solution for a problem I encountered.
I bought a AH-300 from 1989 and it worked well. I noticed the green ' increase input' was lit while no bass was plugged in (and no signal was inserted).
I checked the schematics and found out that all transistors, OpAmps and other circuits were ok.
Then, I noticed that the PCB around power supply section has been discoloured which means it has been very hot.
The 220 Ohm, 5W resistor had been very hot and somehow destroyed the Zener diode next to it.
The Pre-Amp power supply delivered 44V instead of 30V.
This explains why the green LED was continuously lit. As result of the higher voltage, the transistor which drives the LED had enough voltage over its basis-resistor and therefore it opened.
I replaced the Zener diode (33V) and now everything works like a charm.
I also replaced the 220 Ohm resistor by a 9W type and placed it outside the PCB. The copper PCB islands/traces were destroyed by the extensive heat of the resistor, the islands came loose from the PCB.
Now everything runs OK. The green LED only lits when a signal is inserted and the resistor stays warm, but not hot.
Maybe anyone runs into this problem. Here is your solution!
As an electronics engineer, I find the Pre-Amp power supply constructed a bit too cheap. Trace Engineers should have done a better job.....
I bought a AH-300 from 1989 and it worked well. I noticed the green ' increase input' was lit while no bass was plugged in (and no signal was inserted).
I checked the schematics and found out that all transistors, OpAmps and other circuits were ok.
Then, I noticed that the PCB around power supply section has been discoloured which means it has been very hot.
The 220 Ohm, 5W resistor had been very hot and somehow destroyed the Zener diode next to it.
The Pre-Amp power supply delivered 44V instead of 30V.
This explains why the green LED was continuously lit. As result of the higher voltage, the transistor which drives the LED had enough voltage over its basis-resistor and therefore it opened.
I replaced the Zener diode (33V) and now everything works like a charm.
I also replaced the 220 Ohm resistor by a 9W type and placed it outside the PCB. The copper PCB islands/traces were destroyed by the extensive heat of the resistor, the islands came loose from the PCB.
Now everything runs OK. The green LED only lits when a signal is inserted and the resistor stays warm, but not hot.
Maybe anyone runs into this problem. Here is your solution!
As an electronics engineer, I find the Pre-Amp power supply constructed a bit too cheap. Trace Engineers should have done a better job.....